The aim of the proposed research is to obtain information about the role of putative messengers in photoreceptor physiology. Little is known about the events by which the photoisomerization of rhodopsin in disk membranes induces a change in concentration of an intracellular messenger near the site of photon absorption which diffuses to the plasma membrane and modulates the ionic conductance channels. I propose to investigate the actions and interactions of cGMP, Ca++, inosotol trisphosphate (IP3) and diacylglycerol (DG) in excitation and adaptation by modifying their intracellular concentrations in isolated rods. The electrophysiological experiments proposed to investigate the possible roles of these putative messengers are: 1) To determine whether changes in membrane voltage and current induced by IP3 derive from the same ionic mechanism as the light response. 2) To evaluate the possibility that background light can reverse some of the changes in amplitude and time course of the light response induced by the injection of cGMP. 3) To determine if Ca2+ modifies the time course of the light response in cGMP injected cells. 4) To evaluate the possibility that the normal physiological response of the rod can be modified by increased activity of protein kinase C. The experiments proposed in this grant application are intended to clarify the interactions between the putative messengers (cGMP, Ca2+, IP3, DG) and their roles in vertebrate phototransduction. These putative messengers appear to have regulatory functions in many other cells. Therefore, insights into the visual transduction process will have real significance for understanding transduction in other types of receptors and could also provide important perspectives which have general application in neurobiology and endocrinology.